Our present invention relates to a method of making large-volume-hollow bodies at least in part by hydrostatic deformation of a cold-formable metal. More particularly, the invention relates to the fabrication of bodies by hydrostatic shaping, especially for containers such as the fuel tanks of motor vehicles.
The reference to large-volume hollow bodies herein is understood to refer to hollow bodies which may have a capacity of 10 liters or more and can be used, for example, for fuel tanks for automotive vehicles.
In DE 44 36 436, a method of hydrostatic shaping of cold-formable sheet metal is described in which two planar sheets of the metal are fabricated by hydroforming into two matching halves of the large volume body and are then welded together along their equator line and peripherally to produce the hollow body.
This method has the drawback that, because of the need to accommodate after-flow of the metal in the forming of a circumferential flange and the removal thereof and machining in conjunction with the weld seam which is intended to seal the halves together, the cost of the material for the container and the time-consuming process militate against use of the method in mass production.
When efforts have been made to use the earlier method in a mass production sense, because of the peripheral welding, problems have been encountered in reproducibility and reliability of the process.
A process is described in DE 197 19 531 in which a metal blank is welded together after an initial shaping and is then hydraulically deformed to a more or less square or cubic shape. The method described in that publication, however, was found to be applicable only to the formation of bodies of relatively constant cross section which can be fabricated in a single expansion process.
Mention may also be made, in this connection, of EP 0 372 360, DE OS 21 59 852, DE OS 198 51 259 A1 which relate to systems in which hollow structures are formed using weld arrangements and entraining shaping pressure.
It is the principal object of the present invention to provide an improved method of making hollow bodies, especially large hollow bodies as defined, such as motor vehicle fuel tanks, which is applicable even where the hollow body is not of a constant cross section and whereby drawbacks of earlier systems are avoided.
Another object of this invention is to provide a highly economical, reliable and reproducible process for producing hollow bodies, especially for motor vehicle fuel tanks, which obviates drawbacks of the method hitherto mentioned.
These objects are attained, in accordance with the invention, by a method of making a large volume hollow body which comprises the step of:
(a) cutting a blank from a cold-deformable sheet metal having a shape matched to a configuration of the hollow body and a pair of opposite edges between ends of the blank;
(b) shaping the blank generally into the configuration of the hollow body to form a three-dimensional structure smaller than the hollow body, welding the opposite edges together to form a pressure-tight seam along the three-dimensional structure, and closing one end of the three-dimensional structure;
(c) inserting the three-dimensional structure formed in step (b) in a die and expanding the three-dimensional structure in the die to the configuration of the hollow body by hydrostatic shaping through an opposite end of the three-dimensional structure, thereby imparting to the three-dimensional structure a configuration of the die; and
(d) welding to the three-dimensional structure expanded in step (c) at the opposite end thereof a closure to form the structure into the hollow body.
According to a feature of the invention the one end of the three-dimensional structure is closed in step (b) by welding a closure member to the three-dimensional structure with a pressure-tight welding seam.
Prior to hydrostatic expansion of the three-dimensional structure, the latter can be pressed into a flattened oval configuration.
The method of the invention has a number of advantages. The most important advantage is that it fabricates the three-dimensional structure and ultimately the hollow body from a blank which has previously been imparted a shape matched to the configuration of the hollow body and the pressure-tight weld seam which can secure the three-dimensional shape without stress and overlap at the weld seam so that there is little waste of material and the pressure tightness of the weld seam can be reliably and reproducibly made even in a mass-produced product. The hollow body can have a large diameter cross section at one end and a small-diameter cross section at the other end and need not have the same end cross sections. Conical and partially conical and partially cylindrical structures can be fabricated and hydrostatically widened.
The fact that the three-dimensional structure is already of a shape geometrically similar to that of the hollow body which is to be made means that the hydrostatic expansion can be effected in a single step and thus expensive annealing between shaping steps is not required. The closed end may be sealed shut by a closure member welded with a pressure-tight weld seam to the three-dimensional structure even before the expansion of that three-dimensional or tubular structure.
The invention can avoid the need for expensive sealing as is thus required when two openings are provided in the three-dimensional structure before the hydrostatic expansion step. A less expensive die can thus be used than would otherwise be required.
The flattened oval configuration which is imparted to the structure can facilitate insertion of the three-dimensional structure into the die and closing of the die. Frequently when the three-dimensional or tubular structure is not flattened, difficulties are experienced in insertion of the tubular structure into the die or in the closing of the die.
When the three-dimensional structure is filled with the pressure fluid and then pressed into its flattened oval configuration, the formation of folds or the like in the sheet metal material can be avoided.
It has been found to be advantageous, moreover, to include in the three-dimensional structure following the expansion and prior to welding of the closure thereon, to incorporate one or more objects in the three-dimensional structure and thus in the hollow body. Preassembled use such as fuel pumps and the like can be included in this way. The hollow body can be formed during the hydrostatic expansion with an opening for filling or emptying the tank made and it is advantageous to provide the tank with a collar surrounding the opening during the hydrostatic expansion. Before the hydrostatic expansion, a liquid displacement element can be inserted into the three-dimensional structure thereby reducing the time for filling it with the hydrostatic liquid or emptying the hydrostatic liquid from it. The displacement element can be mounted for example on an adapter which fits into the open end of the three-dimensional structure.